Dendrite resistant input bias network for metal oxide semiconductor field effect transistor (MOSFET) devices

ABSTRACT

A circuit includes a high impedance direct current (DC) gate having a DC impedance higher than a maximum impedance DC above which dendrite growth occurs in the circuit, and a low impedance radio frequency (RF) gate having an RF impedance lower than a minimum impedance RF needed to ensure RF stability for the circuit for an application.

CROSS-REFERENCE TO RELATED APPLICATION(S) AND CLAIM OF PRIORITY

The present application is related to U.S. Provisional Patent No. 61/396,365, filed May 26, 2010, entitled “DENDRITE RESISTANT INPUT BIAS NETWORK FOR MOSFET DEVICES”. Provisional Patent No. 61/396,365 is hereby incorporated by reference into the present application as if fully set forth herein. The present application hereby claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent No. 61/396,365.

TECHNICAL FIELD OF THE INVENTION

The present application relates generally to the deposition of ions at localized sites of metal oxide semiconductor field effect transistor (MOSFET) devices and, more specifically, to a system and method for eliminating dendrite growth resulting from the repeated deposition of ions.

BACKGROUND OF THE INVENTION

Silver electromigration (i.e., Ag dendrite) is primarily a direct current phenomenon. Dendrite growth appears as a fern like growth along a surface. Silver dendrite growth requires available silver (exposed silver), a migration medium (moisture), an electric field (gate-source voltage), and a surface (such as beryllium oxide (BeO)).

Silver can electromigrate under non-condensing but humid conditions (i.e., humid electromigration). The rate of electromigration increases directly with relative humidity. In some cases, the moisture needed for electromigration is just a few monolayers of moisture. The positively charged ions of the silver metal move under the influence of an electric field through the moisture paths on the insulator toward the negative conductor where the ions are reduced back to silver metal.

SUMMARY OF THE INVENTION

A circuit is provided. The circuit comprises a high impedance direct current (DC) gate having a DC impedance higher than a maximum impedance DC above which dendrite growth occurs in the circuit, and a low impedance radio frequency (RF) gate having an RF impedance lower than a minimum impedance RF needed to ensure RF stability for the circuit for an application.

A device is provided. The device comprises a circuit. The circuit comprises a high impedance direct current (DC) gate having a DC impedance higher than a maximum impedance DC above which dendrite growth occurs in the circuit, and a low impedance radio frequency (RF) gate having an RF impedance lower than a minimum impedance RF needed to ensure RF stability for the circuit for an application.

A method of manufacturing a circuit is provided. The method comprises determining a maximum impedance direct current (DC) for the circuit above which dendrite growth occurs and determining a minimum impedance radio frequency (RF) needed to ensure RF stability for the circuit for an application. The method further comprises providing an impedance DC for the circuit that is higher than the maximum impedance DC, and providing an impedance RF for the circuit that is lower than the minimum impedance RF.

Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts:

FIG. 1 illustrates silver dendrite growth in a conventional MOSFET;

FIG. 2 illustrates a device having a gate biasing circuit that provides a high impedance DC gate termination and a low impedance RF gate termination according to an embodiment of this disclosure;

FIG. 3 illustrates a method for manufacturing a gate biasing circuit according to an embodiment of this disclosure; and

FIG. 4 shows a negative potential terminal (or source strip) plated “uniformly” with silver due to low current density according to an embodiment of this disclosure.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 through 4, discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged system.

A MOSFET is a high impedance input device. In order for the MOSFET to remain in the active region, a voltage needs to be applied to invert the channel. The voltage that is required to invert the channel typically ranges from 3 to 6 volts of a direct current (DC) voltage. However, the transistors leads for the packages used to carry the MOSFETS are typically braised to the package with a material that contains silver. The package is then plated with gold to protect the silver from outside environmental influences. When a voltage is applied between the gate terminal and the source terminal of the package for the MOSFET to remain in the active region, the electric field that is between the two terminals can allow the silver to migrate in the presence of humidity if the silver is not plated properly by the gold.

FIG. 1 illustrates silver dendrite growth 105 in a conventional MOSFET 100.

In the example shown in FIG. 1, silver ions migrate from a positive gate 101 to a negative source 103 of a MOSFET 100. The ions build upon themselves and eventually create a fernlike structure of dendrite growth 105 along an insulator surface 107 of the negative source 103. The dendrite growth 105 continues to grow from the negative source 103 towards the direction of the positive gate 101, and eventually creates a short or a low impedance path between the positive gate 101 and the negative source 103. Because a MOSFET ideally has infinite input impedance, any leakage or conductive path, such as that created by the dendrite growth 105, causes the MOSFET to de-bias itself. This de-biasing may cause the MOSFET to no longer meet the requirements for a particular system.

This disclosure provides a system and method for eliminating dendrite growth in MOSFET devices.

A low impedance gate to source resistance is normally desired for a gate biasing circuit because MOSFETS typically exhibit a certain amount of instabilities at various frequencies in the RF spectrum. The MOSFET is made stable by picking a certain gate to source resistance. However, having a gate to source resistance that allows a MOSFET to be stable also creates a low impedance DC and radio frequency (RF) gate termination. A low impedance DC and radio frequency (RF) gate termination creates an electric field with enough energy to support dendrite growth. Therefore, while this gate to source resistance may be advantageous for stability, such a resistance is also conducive to dendrite growth.

In an embodiment of this disclosure, external circuit techniques are used to eliminate dendrite growth in MOSFET devices. The probability of repeated deposition at localized sites is reduced by current density reduction within an applied electric field. Current density reduction electroplates the negative terminal “uniformly” with silver, eliminating any dendrite growth across the insulator surface.

FIG. 2 illustrates a device 200 having a gate biasing circuit 210 that provides a high impedance DC gate termination and a low impedance RF gate termination according to an embodiment of this disclosure.

Because the gate impedance of a MOSFET is infinite, a steady or very low impedance DC power supply to bias the MOSFET is not needed. Accordingly, in the example shown in FIG. 2, circuit 210 is biased with an infinite impedance, such as the 1 mega-ohm voltage divider formed by 1 mega-ohm resistors 201 and 203, instead of a low impedance DC power supply. This reduces the amount of energy available for dendrite growth. Furthermore, a lower resistance path to ground formed by a typical gate to source resistance, such as that formed by the 500 ohm resistor 205, is bypassed. Therefore, from a DC perspective, the circuit 210 exhibits a mega-ohm to ground resistance. However, from an RF perspective, the circuit 210 exhibits a resistance parallel to the combination of the 1 mega-ohm resistor 201 and the 500 ohm resistor 205, which is effectively 500 ohms. The DC block 207 ensures that the circuit 210 is shorted at RF and infinite at DC.

The impedance DC of the circuit 210 is high in the sense that the impedance DC is higher than a maximum impedance DC for the circuit 210 above which dendrite growth occurs. The impedance RF is low in the sense that the impedance RF is lower than a minimum impedance RF needed to ensure RF stability for the circuit 210 for a particular application.

Accordingly, the high impedance DC and low impedance RF gate biasing circuit 210 as shown in FIG. 2 eliminates silver dendrite growth while maintaining acceptable RF stability.

Of course one of ordinary skill in the art would recognize that circuit 210 is only one example of a high impedance DC and low impedance RF gate biasing circuit according to this disclosure and that other circuit designs are within the scope of this disclosure.

FIG. 3 illustrates a method 300 for manufacturing a gate biasing circuit according to an embodiment of this disclosure.

As shown in FIG. 3, method 300 includes determining a maximum impedance DC for a circuit above which dendrite growth occurs (block 301). Method 300 also includes determining a minimum impedance RF needed to ensure RF stability for the circuit for a particular application (block 303). Method 300 further includes providing an impedance DC for the circuit that is higher than the maximum impedance DC (block 305), and providing an impedance RF for the circuit that is lower than the minimum impedance RF (block 307).

FIG. 4 illustrates a MOSFET 400 having a negative source plated “uniformly” with silver according to an embodiment of this disclosure.

As shown in FIG. 4, silver ions migrate from a positive gate 401 to a negative source 403 of MOSFET 400. However, unlike MOSFET 100, the ions in MOSFET 400 form a uniform layer 405 along an insulator surface of the negative source 403. As a result, the negative potential terminal in FIG. 4 has a reduced probability for dendrite deposition. Once the entire exposed available silver is depleted, no possibility of silver dendrite growth exists, regardless of the gate bias supply impedance.

Accordingly, moving any exposed available Ag material from within the leadframe and attaching the material with low current density prevents localized repeated Ag deposition (Ag dendrites) from causing device failure.

Although the present disclosure has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present disclosure encompass such changes and modifications as fall within the scope of the appended claims. 

1. A circuit comprising: a high impedance direct current (DC) gate having a DC impedance higher than a maximum impedance DC above which dendrite growth occurs in the circuit; and a low impedance radio frequency (RF) gate having an RF impedance lower than a minimum impedance RF needed to ensure RF stability for the circuit for an application.
 2. The circuit in accordance with claim 1 wherein the circuit is biased with an infinite impedance.
 3. The circuit in accordance with claim 2 wherein the infinite impedance is created by a 1 mega-ohm voltage divider.
 4. The circuit in accordance with claim 1 wherein the circuit includes an infinite impedance path to ground and a lower resistance path to ground.
 5. The circuit in accordance with claim 4 wherein the circuit exhibits a mega-ohm to ground resistance with regard to DC, and exhibits the lower resistance path to ground with regard to RF.
 6. The circuit in accordance with claim 4 wherein the lower resistance path to ground is created by a 500 ohm resistor.
 7. The circuit in accordance with claim 1 further comprising a DC block configured to provide a short at RF and an infinite resistance at DC.
 8. A device comprising: a circuit comprising: a high impedance direct current (DC) gate having a DC impedance higher than a maximum impedance DC above which dendrite growth occurs in the circuit; and a low impedance radio frequency (RF) gate having an RF impedance lower than a minimum impedance RF needed to ensure RF stability for the circuit for an application.
 9. The device in accordance with claim 8 wherein the circuit is biased with an infinite impedance.
 10. The device in accordance with claim 9 wherein the infinite impedance is created by a 1 mega-ohm voltage divider.
 11. The device in accordance with claim 8 wherein the circuit includes an infinite impedance path to ground and a lower resistance path to ground.
 12. The device in accordance with claim 11 wherein the circuit exhibits a mega-ohm to ground resistance with regard to DC, and exhibits the lower resistance path to ground with regard to RF.
 13. The device in accordance with claim 11 wherein the lower resistance path to ground is created by a 500 ohm resistor.
 14. The device in accordance with claim 8 further comprising a DC block configured to provide a short at RF and an infinite resistance at DC.
 15. A method for manufacturing a circuit, the method comprising: determining a maximum impedance direct current (DC) for the circuit above which dendrite growth occurs; determining a minimum impedance radio frequency (RF) needed to ensure RF stability for the circuit for an application; providing an impedance DC for the circuit that is higher than the maximum impedance DC; and providing an impedance RF for the circuit that is lower than the minimum impedance RF.
 16. The method in accordance with claim 15 wherein the circuit is biased with an infinite impedance.
 17. The method in accordance with claim 16 wherein the infinite impedance is created by a 1 mega-ohm voltage divider.
 18. The method in accordance with claim 15 wherein the circuit includes an infinite impedance path to ground and a lower resistance path to ground.
 19. The method in accordance with claim 18 wherein the circuit exhibits a mega-ohm to ground resistance with regard to DC, and exhibits the lower resistance path to ground with regard to RF.
 20. The method in accordance with claim 18 wherein the lower resistance path to ground is created by a 500 ohm resistor.
 21. The method in accordance with claim 15 further comprising a DC block configured to provide a short at RF and an infinite resistance at DC. 